In our studies of the development and evolution of immune mechanisms, we are addressing several specific issues of phylogenetic importance. (1) We are conducting comparative studies of T and B lymphocytes in a reptile, the snapping turtle, in order to assess the species' cellular responsiveness to mitogens and antigens, the cellular basis of immunorecognition and cell diversity. (2) We are studying the cellular mechanisms involved in antigen trapping, cellular diversification and induction of stem cell maturation in the avian bursa from a developmental standpoint. (3) We are investigating the immune capacity (cell-mediated and humoral) of a hibernating vs. non-hibernating mammal (bat) and comparing these responses to those of ectothermic vertebrates at various environmental temperatures. (4) We are studying the cellular basis of allograft incompatibility in corals.